The present invention relates generally to mass digital data storage systems, and, more particularly, to methods for non-volatile memories systems to determine and supply the user information on their remaining life.
The use of non-volatile memory systems such as flash memory storage systems is increasing due to the compact physical size of such memory systems, and the ability for non-volatile memory to be repetitively reprogrammed. The compact physical size of flash memory storage systems facilitates the use of such storage systems in devices, which are becoming increasingly prevalent. Devices that use flash memory storage systems include, but are not limited to, digital cameras, digital camcorders, digital music players, handheld personal computers, and global positioning devices. The ability to repetitively reprogram non-volatile memory included in flash memory storage systems enables flash memory storage systems to be used and reused.
Although non-volatile memory or, more specifically, non-volatile memory storage cells within flash memory systems may be repetitively programmed and erased, each cell or physical location may only be erased a certain number of times before the cell wears out. In some systems, a cell may be erased up to approximately ten thousand times before the cell is considered to be unusable. In other systems, a cell may be erased up to approximately one hundred thousand times or even up to a million times before the cell is considered to be worn out. When a cell is worn out, thereby causing a loss of use or a significant degradation of performance to a portion of the overall storage volume of the flash memory system, a user of the flash memory system may be adversely affected, as for example through the loss of stored data or the inability to store data.
The wear on cells, or physical locations, within a flash memory system varies depending upon how often each of the cells is programmed. If a cell or, more generally, a memory element, is programmed once and then effectively never reprogrammed, the wear associated with that cell will generally be relatively low. However, if a cell is repetitively written to and erased, the wear associated with that cell will generally be relatively high. As logical block addresses (LBAs) are used by hosts, e.g., systems which access or use a flash memory system, to access data stored in a flash memory system, if a host repeatedly uses the same LBAs to write and overwrite data, the same physical locations or cells within the flash memory system are repeatedly written to and erased, as will be appreciated by those of skill in the art.
When some cells are effectively worn out while other cells are relatively unworn, the existence of the worn out cells generally compromises the overall performance of the flash memory system, which can be manifested in blocks which fail to program or erase (and eventually the system can run out of the blocks required to store data), or that have data unreliable upon reading. Although error correction techniques can be used, error correction takes time and, at some point, the system may reach a point when data storage reliability is unacceptable due uncorrectable errors or a high level of correctable errors causing excessive command (both read and write) execution latency. The system can reach this state even before it runs out of spare blocks due to program or erase block failures. In addition to degradation of performance associated with worn out cells themselves, the overall performance of the flash memory system may be adversely affected when an insufficient number of cells which are not worn out are available to store desired data. Often, a flash memory system may be deemed unusable when a critical number of worn out cells are present in the flash memory system, even when many other cells in the flash memory system are relatively unworn.
Although there are number of methods to increase the life of such memories, it would advantageous for the user to know how much life the memory has left before it is worn out and data is lost or it lapses into a read only mode. US patent application publication number US 2003/0002366 A1 describes a memory that generates a warning when number of spare storage areas is smaller than a predetermined value, but this warning is only given after the memory has reached this state and is based only the number of spare storage remaining areas at that time. It provides no evaluation based on a linearly increasing measure of wear nor indication of the remaining life as time. Consequently, there is room for improvement in providing the user with information on the state of memory and its remaining lifetime.